Hola,
No es nada extraño. Esa funcion pertenece a una extension, la bcmath. Y aqui esta lo que pone como instalacion en el manual:
Cita: Installation
In PHP 4, these functions are only available if PHP was configured with --enable-bcmath. In PHP 3, these functions are only available if PHP was NOT configured with --disable-bcmath.
The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.
Es decir, que parece que hay que habilitar el uso de la extension en la compilacion.
Asi que tu script dependera de la configuracion de PHP.
Saludos.